1. Field of the Invention
The present invention relates to an optical disk having a sector structure and an apparatus for recording and reproducing the same. More particularly, it relates to an optical disk on which data are recorded in a plurality of sectors with error detection and correction coding performed between the sectors and to an optical disk reproducing apparatus for reproducing the data from such an optical disk.
2. Description of the Related Art
For an optical disk on which data are divided into sectors ranging from 512 bytes to 2 kilo-bytes to be recorded, coding for detecting and correcting errors is performed on a sector basis when the data are recorded so that the data in each sector will be properly reproduced. However, since the error detection and correction codes are provided on the basis of sectors ranging from 512 bytes to 2 kilo-bytes, it is difficult to accommodate a sufficient interleave length. As a result, in the case of a long error affecting the entire sectors, correction of such an error has been impossible and proper data reproduction has been difficult in some cases. In the case of a write once type or rewritable type optical disk, it has become common to perform the so-called read-verify operation wherein data are reproduced immediately after recording in order to verify that the data are properly reproduced and, if they are not properly reproduced, to perform a substitution process wherein the data are rerecorded in substitute sectors.
For a read only type optical disk used for reproduction such as a computer program and a dictionary for a word processor wherein code data are recorded in advance, unlike the recordable optical disks such as the write once type and rewritable type, the read-verify process and the substitution process as described above can not be performed. Therefore, the ability of correcting errors itself must be improved to prevent correction from being disabled.
As methods for improving the error correcting capability, there are a method wherein the error correction code for each sector is enhanced and a method wherein recording is performed on sectors having the same structure as that of the recordable type sectors by adding parity sectors wherein error detection and correction coding has been performed between the plurality of sectors. When compatibility between the recordable type and reproduction-only type is considered, the method using parity sectors is preferable because the structure of sectors can be the same according to this method. As disclosed in U.S. Pat. No. 4,949,326 and U.S. Pat. No. 5,077,720, in an optical disk or an optical disk reproducing apparatus employing the parity sectors, for example, the overall parity for the data sector of one track is prerecorded as one parity sector.
For optical disks of the recordable type, error correction is performed with only the error correction code for each sector. In most optical disks of the reproduction only type, error correction is also performed using the error correction code for each sector and, only when error correction of the sectors is disabled, error correction is performed using the parity sectors which have been configured as error correction codes between the sectors.
The use of error correction codes having high error-correcting capability such as Reed-Solomon codes having a large minimum distance for such inter-sector coding which is not normally used and is added only for security purposes results in a significant increase in hardware and consequently in the manufacturing cost. It has been common to employ a simple structure wherein the error correction code for each sector used for both the recordable type and reproduction type is relatively strong and one overall parity sector is provided for each track as a parity sector.
Optical disks are characterized by their ability of high-density recording and data are recorded thereon by forming recording pits which are not deeper than 1 um on tracks provided at pitches of about 1.5 um. Further improvement of recording density is currently pursued by, for example, making the wavelengths of the semiconductor lasers shorter. As recording density is improved, scratches, dust and the like on the media become more harmful to the reliability of optical disks. Specifically, as recording density is improved, one scratch of dust may disable a plurality of sectors from being corrected simultaneously. This has resulted in a problem that reliability in data reproduction has become insufficient for conventional reproduction type optical disks utilizing simple overall parity as parity sectors. Such sectors which can not be corrected constitute a burst error for sectors which are consecutive in the direction of the tracks or the radial direction of the disk, and a sufficient error correcting capability can not be obtained even for error correction codes having a high error correcting capability such as a Read-Solomon code which is capable of correcting errors in a plurality of sectors for inter-sector coding.